The present invention relates to a system for diagnosing troubles of an exhaust gas recirculation system for an automotive engine.
If a trouble or malfunction occurs in the exhaust gas recirculation (hereinafter called EGR) system, amount of NOx in exhaust gases increases. Accordingly, a diagnosing system is provided for the EGR system so as to alarm the trouble or malfunction of the EGR system.
Japanese Patent Laid Open 55-123345 discloses a diagnosing system for the EGR system which produces an alarm signal when opening degree of an EGR valve deviates from a desired value over a predetermined allowable range. In the EGR system, the carbon in the recirculated gas is liable to stick to an inner wall of an exhaust gas recirculation passage and a port of the EGR valve to reduce inner diameters of the passage and port, which reduces the rate of the EGR. It should be noted that the EGR system must operate to keep a proper EGR rate. The above mentioned prior art can not diagnose such a trouble, if the EGR valve operates normally.